With the recent trend to a front wheel front drive (FF) system of automobiles, the use of a CVJ which can evenly transmit the power of an engine to rotate right and left wheels at a given velocity has been increasing rapidly. In particular, a tripod type CVJ has a structure shown in FIGS. 1 and 2, in which outer ring 11 has on the inner side thereof three cylindrical grooved tracks 12 in the axial direction at equally divided angles, and tripod member 13 fitted into outer ring 11 has three axial feet 14. Spherical-surfaced roller 15 is fitted onto the outer side of each axial foot. A number of needle bearings 16 are fitted between spherical-surfaced roller 15 and axial foot 14 to bear spherical-surfaced roller 15 in such a manner that the roller may rotate and slide in the axial direction. Spherical-surfaced roller 15 is fitted into grooved track 12. Such a tripod type CVJ is apt to generate a resisting force in its axial direction because of the involvement of reciprocating rolling and sliding between cylindrical groove 12 and spherical-surfaced roller 15 on revolution. This force (hereinafter referred to as induced thrust force) causes vibration, such as a shudder of the car body upon starting off and acceleration.
Therefore, grease to be applied to a tripod type CVJ is keenly demanded to reduce frictional resistance of the sliding part. Grease having excellent lubricating action not only reduces the above-described vibration but also suppresses frictional heat generation, thereby leading to improved durability of the CVJ.
In order to meet the above demand, cases are increasing in the market in which urea grease having high heat resistance and excellent frictional wear characteristics is used. The grease compositions disclosed in JP-A-2-20597 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-B-5-79280 (the term "JP-B" as used herein means an "examined Japanese patent publication") may be mentioned as typical examples of the art.
The grease composition of JP-A-2-20597 comprises a base oil, a thickening agent comprising a diurea compound, a urea-urethane compound, and a diurethane compound, a sulfurphosphorus type extreme pressure additive consisting of (1) an alkali metal borate hydrate, (2) at least one molybdenum compound selected from a molybdenum dithiophosphate (Mo-DTP), molybdenum dithiocarbamate (Mo-DTC), and molybdenum disulfide (MoS.sub.2), and (3) at least one compound selected from the group consisting of sulfated fat and oil, polysulfide, a phosphate, a phosphite, a thiophosphate, and zinc dithiophosphate.
The grease composition of JP-B-5-79280 is for a CVJ and comprises urea grease and, as additives, Mo-DTC and Mo-DTP or a combination of these organomolybdenum compounds and zinc dithiophosphate (Zn-DTP).
However, when these conventional grease compositions are applied to a tripod type CVJ, they are not regarded as satisfactory because vibration still occurs in the shaft, although the induced thrust force is lower than that generated in using commercially available grease.